This invention relates to soil-compacting devices of the kind comprising a body containing a vibratory mechanism and arranged to be introduced into or to form a hole in the ground, such as may be required to receive piling or other building foundation works.
Such devices are commonly of elongate form to facilitate the compaction process and for better guidance of the device along a straight path. To obtain a desirable overall length without attenuating the effect of the vibratory mechanism unduly, it is known to couple the body containing the mechanism with a follower that forms an upward extension of the body, the coupling being designed so as to reduce the transmission of vibrations from the vibrator body to the follower.
The coupling is required to support the vibrator body from the follower, at least when the device is suspended above a hole, and it must also not allow the body to rotate freely because pipelines and cables extending through the follower into the body, e.g. to power the vibratory mechanism, would then become twisted. It is not easy to combine these requirements with the need to minimize the transmission of vibrations from the body to the follower and also to provide sufficient internal space in the coupling for the passage of pipelines and cables. The result has been a compromise in which some of the desired results are sacrificed.
Thus, UK patent specification No. 1 278 696 describes the use of a series of universal couplings to connect the vibrator body and follower tube in a way that allows relative lateral displacements, but the couplings are bulky and restrict substantially the space available for the required pipelines and cables and the components of the universal couplings are susceptable to accelerated wear in the dirty working conditions to which they are exposed.
In UK patent specification No. 987 312 there is described an arrangement in which the body is suspended through resilient rings bonded to the body and the follwer, at frusto-conical interfaces. This arrangement relies on the supposition that the body moves as a conical pendulum about a vertical centre defined by the apex of the conical surface normal to said interfaces, but the restraints imposed by the walls of the hole in which the compacting device is working can prevent this, and since soil conditions will normally vary consierably and unpredictably it is impossible to make allowance for their effect. Any such deviations will render the resilent rings ineffective for their intended function of isolating the follower from the vibrations.
The known construction also requires that the vibrator body can be suspended from the follower through the rings,and therefore through their bonded faces. Even when the weight of the vibrator body is taken by the bottom of the hole, it is still necessary for the bonding to allow for stress reversals leading to very substantial tensile stresses because of the movements of the coupling when the body oscillates angularly with respect to the follower. It is difficult to ensure that the bonding has sufficient strength to withstand these various stresses and not only is it expensive to rely upon bonding of such high quality but it is also likely to require frequent repair and maintenance. In fact, quite apart from strength requirements, the bonding is also required to maintain a complete seal between the body and the follower as the device is necessarily exposed to extreme working conditions and would otherwise be rendered inoperative by the ingress of foreign matter and moisture.
It is an object of the present invention to provide an improved form of coupling for a vibratory soil-compacting device that avoids the aforementioned difficulties.